Conventional state of the art propulsion systems for large civil aircraft typically include one or more gas turbine engines placed under the wings of the aircraft. However, some studies have indicated that so-called distributed propulsion, which involves having numerous smaller propulsion units preferentially arranged around an aircraft, may provide some significant benefits in terms of noise reduction and fuel efficiency when compared with the current state of the art technology.
One option for a distributed propulsion system is to have numerous electrically powered fan units located around the aircraft. However, early studies by the applicant have indicated that novel electrical technology will be required to implement such a distributed electrical system.
One such technology is the creation of a superconducting system to provide the electrical power to the fan units so as to try and reduce the weight of the electrical system.
The concept of using a superconductor for providing electrical power is well known. A superconductor conducts electricity without loss, that is, with zero electrical resistance. In order to be superconducting, current state of the art superconductor materials must be maintained below a critical temperature, current density and magnetic field. If any of the critical limits are exceeded then the superconductor is said to “quench”, at which point it reverts to its “normal” electrical (and magnetic) properties.
For example, in the case of Yttrium Barium Copper Oxide, YBCO, the critical temperature is 93K; the upper critical magnetic flux density field is 120 T for a field perpendicular and 250 T for a field parallel to the copper oxide planes, and the critical current density is 30 GA m−2. The so-called “supercurrent”, that is the current that flows in the super conductor when in its superconducting state, flows in a very thin layer at the surface of the superconductor, typically 800 nm (the London Depth). However, the critical current density reduces with applied magnetic field and also will reduce as the temperature approaches the critical temperature.
In the case of ceramic superconductors the quenched electrical resistance can be very high. Hence, it is possible, and known, to provide a switch where an applied magnetic field is used to control the superconducting state of a superconductor and thus switch it between operating points having high and low (zero) resistance.
FIG. 1 shows the basic concept for a cryotron 10 which uses an electrical coil 12 wrapped around a length of superconductor 14. The superconductor current, Ig, flows until a direct current, Ic, of sufficient magnitude to produce a quenching magnetic field flows through the electrical coil 10. Once this occurs, the resistance increases until there is negligible current flow, thereby providing a switch.
The present invention seeks to provide a superconducting switch of general application but which may preferably be used in a distributed propulsion system of an aircraft.